1. Field of the Invention
The present invention relates to an illumination light source that employs a coherent light source, and an image display apparatus that employs this illumination light source.
2. Description of the Related Art
In recent years, a laser is used as a light source of optical image display apparatuses in many cases. Generally, a light emitted from a laser has high directivity, and therefore, it is expected that the light use efficiency be improved. In addition, a monochromatic property of the laser light realizes a wide color reproduction range that is required in the image display apparatuses. Therefore, lasers are considered useful as an illumination light source. On the other hand, because the laser light has high coherence, if a laser is used as a light source of an image display apparatus, there is a problem that dot patterns called a speckle noise due to interference of the light is caused. This is caused because minute irregularity in devices in an illumination optical system, light bulbs, in devices in a projection optical system, or on a screen causes the phases of the light beams that have passed through different points on a device surface to shift by an amount corresponding to the irregularity, and those light beams that are coherent with each other form an interference pattern on an image surface. Because surface precision of the devices is limited, when a light source having high coherence is used, influence of the speckle noise is always a problem. With the recent advance in a laser technology, development of a semiconductor laser that is compact and high-power, and that outputs high quality beams, and the like is active, and it is expected that a laser is increasingly used as a light source of image display apparatuses in future. Therefore, establishment of a speckle noise suppression method is demanded.
The problem of the speckles in image display apparatuses in which a laser is used as a light source can be solved by reducing only coherence of a laser beam while maintaining the property of the laser beam, such as the monochromaticity and the high directivity. The coherence of the laser beam can be reduced by largely disarranging its equi-phase plane. As a method of reducing the coherence of the laser beam, a method in which a coherent light beam generated by a single laser (resonator) is divided into a plurality of light beams that are incoherent with each other and the light beams are synthesized, or a method in which a plurality of coherent light beams that are incoherent with each other generated by different lasers (resonators) are synthesized has been used.
As a technique based on the former method, a technique in which a plurality of juxtaposed element lenses (microlens array) are used (see, e.g., Japanese Patent Application Laid-open No. 2000-268603) and a technique in which a fiber bundle composed of a plurality of optical fibers having different lengths is used (see, e.g., Japanese Patent Application Laid-open No. H11-326653) have been proposed. In these techniques, light emitted from a laser is divided into a plurality of light beams using those devices to make a difference in an optical distance between the light beams larger than the coherence length of the light. Thus, the light beams become incoherent with each other, thereby reducing the coherence of the light source as a whole.
As a technique based on the latter method, an exposure semiconductor laser light source that includes a plurality of semiconductor lasers having different wavelengths and an optical fiber corresponding to each of the semiconductor lasers, and that emits a low coherent light from the optical fibers has been proposed (see, e.g., Japanese Patent Application Laid-open No. 2004-146793). In this technique, because the light beams emitted from the semiconductor lasers have low coherence, the coherence of a light obtained by synthesizing the light beams becomes low as a whole.
According to the above techniques, a low coherent light can be generated that is constituted by a plurality of light beams incoherent with each other. Furthermore, the coherence of such a synthesized light beam can be sufficiently reduced by increasing the number of devices. However, because light loss is large in light division using a microlens array or a fiber bundle as in the former method, high light use efficiency cannot be achieved. On the other hand, when a plurality of semiconductor lasers having different wavelengths are used as a light source as in the latter technique, wavelength intervals are required to be large to sufficiently reduce the coherence. Therefore, a wavelength range in the light source is to be wide, resulting in degradation of the monochromaticity. In addition, in the above techniques, to further reduce the coherence, a large number of fibers, light division devices, and semiconductor lasers are required. While the increase of the devices can enhance the image quality such as color and brightness, it is not suitable for image display apparatuses for which compact size of an illumination device and cost reduction are demanded.
As described above, the conventional speckle reducing methods still have problems to be solved in terms of light use efficiency, cost, and compact size of the apparatuses.